The present invention relates to a method for making a bearing bushing without machining specifically the type used for the support of trunnions in universal joints. These bearing bushings which are shaped in a die typically comprise a cylindrical sleeve section and a bottom section closing the sleeve section off at one axial end thereof and a peripheral edge with a very small cross sectional radius formed between the outer peripheral surface of the sleeve section and the outer axial end face of the bottom section.
The prior art discloses a process for forming bearing bushings in this fashion wherein first a blank with essentially a flat bottom section is produced which is subsequently deformed in the radial direction at its outer rim. This known process which is disclosed in German Preliminary Application 1,575,502 has the disadvantage that the material of the bearing bushing flows irregularly at its rim particularly in the transition zone between the outer peripheral surface of the sleeve section and the outer axial end face of the bottom section since the drawing and supporting rim of the die are disposed in an offset engagement at the periphery because of the inevitably small guiding and shaping defects in the die. This non-uniform flow of material is particularly critical in bearing bushings used as the support of trunnions in universal joints where the edge should be shaped as a sharp edge at the transition zone between the outer surface of the sleeve section and the outer axial end face of the bottom section. In accordance with the known process, harmful internal stresses, cracks and waviness commonly referred to as the "orange peel effect" are created near the peripheral edge which impairs the strength of the bearing bushing. This type of damage and the non-uniformity of the material of the bearing bushing can ultimately lead to fracture of the bearing bushing wall when it is axially stressed, for example, by the front surface of the trunnion of a universal joint, which axially abutts the bottom section of the bearing bushing and transmits bearing pressure and bending stresses to the transition zone between the outer peripheral surface of the sleeve section and the outer axial end face of the bottom section.
The present invention provides a method for making bearing bushings which eliminates some of the disadvantages and drawbacks of the prior methods discussed above. Essentially the present invention is based on the objective of providing a method for producing a bearing bushing without machining. To this end, the method steps include molding a blank with a sleeve section and a bottom section disposed at one axial end of the sleeve section and a bottom section disposed at one axial end of the sleeve section and uniformly running axially inward to a central tip and then flattening from the inside at least the central tip of the bottom section by supporting and a corresponding forging at the outside surface of the bottom section in the area of the peripheral edge between the outer surface of the sleeve section and the outer surface or face of the bottom section. Bearing bushings made by this method exhibit high strength and accuracy and it is possible to manufacture bearing bushings in accordance with the present method in a highly economical manner in spite of shaping a peripheral edge with an extremely small cross sectional radius.
Considering the method of the present invention more specifically, the bottom section is initially provided with a central conical tip which is pressed axially inwardly, thereafter flattening the central tip of the blank while supporting an annular zone of the bottom section in the area of the peripheral edge. In this manner the forging pressure is always applied centrally, that is, at the central tip of the bottom section and as a result the forging and bending forces acting in the bottom section are conducted uniformly outwardly to the peripheral edge of the blank. In this manner a uniform flow of the material takes place in the zone of the peripheral edge between the outer peripheral surface of the sleeve section and the outer face of the bottom section to form a relatively sharp fissure-free peripheral edge. It has been found that this uniform material flow is not disturbed even when inevitably small radial offsets or misalignments are present between the tool elements which press from opposite sides at the tip of the bottom section and in the zone of the peripheral edge, for example, as a result of guidance play for the press and support ram.
During cold forming or molding of the peripheral edge, relatively low internal stresses are induced in the material since only slight relative movements take place during forging between the blank material and the pressure surfaces of the forging or molding tools. The friction forces and the resulting active forces exerted by the tools are therefore small so that an advantageously long useful service life of the tools results.
Even small internal stresses which may occur in the zone of the peripheral edge are distributed in rotational symmetry because of the concentric flow of the material in the zone of the peripheral edge even in large sized bearing bushings. For this reason, the bearing bushing so formed is truly concentric about its central axis and it has been found that oval deformation of the bearing bushing when it is taken from the supporting die does not occur. Thus, the bearing bushing produced in accordance with the method of the present invention has a very accurate shape.
In accordance with other specific features of the method set forth above, the material flowing in the wall of the bottom section during the shaping operation and specifically when the central tip is flattened on an annular zone of a supporting tool and is conducted or moved in rotational symmetry in this zone against the supporting tool and there to the peripheral edge into the annular space between the peripheral edge of the blank and the supporting tool and fills this space completely and uniformly. In one of the forging dies illustrated, a conical depression is provided in the bottom wall of the bearing bushing and this contributes to the economy of the process by utilizing tools having simple geometric shapes.
Additional economies can be effected by rounding the central tip of the bottom section and combining that operation with a final deep drawing of the sleeve section of the blank.